Den Mothers: Bears shift dens as ice deteriorates

Pregnant polar bears in northern Alaska are now more likely to dig their birthing dens on land or landbound ice than on the offshore ice they once used, according to 20 years of records.

This landward trend probably reflects the decline of the sea-ice habitat these bears have traditionally relied on, says Steven Amstrup of the U.S. Geological Survey in Anchorage, Alaska. He and his colleagues found the trend toward coastal denning in a long-term data set that records polar bear movements.

These bears don't hunker down in dens to survive winter. They specialize in prowling ice to ambush seals. Only pregnant females dig dens, where they spend winters giving birth and nursing cubs.

Alaskan polar bears

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